BiMOS (bipolar-MOS) and BiCMOS (bipolar-complementary MOS) semiconductor devices are devices which utilize both bipolar and MOS transistors. BiCMOS refers to devices having N-channel (NMOS) and P-channel (PMOS) transistors, or in other words complementary transistors, while BiMOS typically has one of either NMOS or PMOS transistors. BiMOS and BiCMOS devices are becoming increasingly popular, in part because MOS transistors are reaching performance and manufacturing limitations as the size of the transistors are reduced. For example, smaller transistors require thinner gate oxides. However, as the gate oxide thickness is decreased it is more difficult to grow oxides which are free of defects. As another example, doping concentration is typically increased as transistor gates are scaled; however, increased doping results in decreased carrier mobility. One way of overcoming some of these scaling and performance limitations is to combine MOS transistors with bipolar transistors to form BiMOS or BiCMOS devices. BiMOS and BiCMOS devices have advantages associated with bipolar transistors, such as faster speed and higher current driving capability, in addition to advantages associated with MOS transistors, such as low power consumption. Unfortunately, combining bipolar and MOS transistors also has disadvantages, two of which are an increase in manufacturing complexity and an increase in overall device size.
As a way of reducing the size of BiMOS and BiCMOS devices and reducing the number of manufacturing steps necessary to fabricate such devices, semiconductor manufacturers have begun to merge various components of bipolar and MOS transistors. Historically, BiMOS and BiCMOS devices utilized bipolar transistors which were extensively isolated from MOS transistors. However, isolating the two types of transistors resulted in a rather large overall device size. As the need for smaller devices grew, semiconductor manufacturers realized that extensive isolation between bipolar and MOS transistors was not necessary and began to form the two transistors closer and closer together. Beyond having closely spaced bipolar and MOS transistors, manufacturers also found that it is feasible to merge the two types of transistors such that components are shared to further reduce overall device size. For example, one known BiMOS device forms an N-well which functions both as a collector for an NPN bipolar transistor and a bulk region for a PMOS transistor. Another commonly shared component is a P.sup.+ diffusion region which serves as an extrinsic base for an NPN bipolar transistor and as a source/drain for a PMOS transistor. Similarly, complementary transistors, namely PNP bipolar transistors and NMOS transistors have been made with shared components which are comparable to those mentioned above.
While some components of bipolar and MOS transistors are known to be shared, there is still an ongoing need for semiconductor manufacturers to reduce the size of semiconductor devices. To achieve the advantages of both bipolar and MOS transistors while satisfying demand for small devices, it is necessary to merge the two types of transistors beyond levels which are known in the art.